The present invention relates to a focus detection circuit for detecting when the focal spot of a focussed light beam is in focus on a surface towards which the light beam is directed.
It is known in optical scanning probes to focus a light beam from a semiconductor laser onto a surface using a movable objective lens. The reflected light is re-focussed inside the probe onto a focus detector consisting of a pair of photo-diode detectors, the output of which controls a lens tracking mechanism which maintains the focal point on the surface. The design is such that a displacement of the object surface in the direction of the light beam results in a change in the distribution of light energy falling on the photo-diodes. This provides a focus error signal which is used to move the lens in a direction such as to reduce the focus error signal to zero, at which point the focal point of the light beam is back on the surface.
The above-described tracking mechanism relies on the fact that the focal point is always at the same distance from the lens, when using a collimated or coherent light source, so that the movement of the lens which is required to bring the light spot back into focus on the surface equates to the movement of the surface.
It is also known to provide an optical trigger probe (for example a probe described in an article in OEM DESIGN published in Oct. 1986) which may be moved towards a surface and provides a signal when a beam emitted by the probe comes into focus on the surface.